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This paper describes the operation of a field emitter array (FEA) as the electron source of a traveling-wave tube (TWT) amplifier. Issues of beam control and focus at high current density and low magnetic field are addressed as well as issues relating to the inherent high emittance of the FEA beam and cathode protection from ion bombardment. Large signal, nonlinear RF-modulated FEA-TWT interaction simulations show circuit efficiencies that approach 50%, even for minimal bunching of average-to-peak current ratios of 0.7-0.9. Direct radio frequency (RF) modulation at the cathode is predicted to significantly improve linearity in the high-efficiency regime as well as reduce harmonic power levels. An unmodulated C-Band FEA-TWT was built to test the focusing approach as well as the robustness of the emitters in an operating vacuum device. The device uses a 1-mm diameter Spindt emitter with a custom-designed electron gun and helix circuit. The FEA-TWT has operated to date with a maximum current of 91.3 mA and shows 99.5% transmission under both drive and no-drive conditions. Output power of the device is 55.0 W at 1.5 GHz with a saturated gain of 23.4 dB and efficiency of 17%, and performs as predicted by simulation. During all operation, the FEA emission appears extremely stable, with no temporal variations observed at any time.